The invention relates to seafloor electromagnetic (EM) surveying for oil and other hydrocarbon reserves.
Geophysical methods for mapping subterranean resistivity variations by various forms of EM surveying have been in use for many years [1, 2, 3, 10]. In these methods, electric field detectors are placed on the seafloor at carefully chosen positions at ranges up to about 10 km from an electromagnetic source. Detector signals measured at the detectors are sensitive to variations in subterranean strata configuration resistivity beneath the area being surveyed. However, EM surveying was not widely thought of as a technique that could be applied to finding hydrocarbon reservoirs.
More recently, it was proposed to use EM surveying to find hydrocarbon reservoirs. An early proposal by Statoil was to use the vertical current flow components to detect hydrocarbon layers [4, 5], since it is these components that are sensitive to the presence of a thin resistive layer. This was based on the understanding that a subterranean strata configuration that includes a resistive hydrocarbon layer embedded within less resistive sediments will give rise to a measurable enhancement of the electric field amplitude compared to a subterranean strata configuration comprising only water-bearing sediments. The Statoil proposal was to collect data from detector locations which are in-line with (i.e. end-on to) the axis of a horizontal electric dipole (HED) antenna so that the galvanic mode, that should be most sensitive to the presence of a buried high resistivity layer, dominates. However, it was established that the Statoil method could not provide reliable results, since the in-line data collected is incapable of distinguishing between a thin buried hydrocarbon layer of high resistivity situated in less resistive strata, on the one hand, and a non-hydrocarbon bearing rock formation in which the strata exhibits increasing resistivity with depth, on the other hand, the latter being a common feature of many large scale sedimentary structures.
It was then proposed to use the EM surveying method according to Sinha [12] for finding hydrocarbon reservoirs [9, 13] and it was then confirmed that this method works well in practice for finding hydrocarbon reservoirs [6, 7]. The essence of the Sinha method is to normalise the in-line data with equivalent data for the same source-detector pair locations collected in an orthogonal geometry where the inductive mode dominates the response, referred to as a broadside geometry. In the broadside geometry the axis of the HED dipole antenna of the source is perpendicular to a line between the detector and source. EM surveying of a hydrocarbon reservoir applying the Sinha method is now described in more detail.
Survey data is collected by using a surface vessel to tow a submersible vehicle carrying a HED antenna over a survey area. The HED antenna broadcasts a source electromagnetic signal into the seawater. Detectors are located on the seafloor over the survey area and measure a signal in response to EM fields induced by the HED antenna. The amplitude of the detector signals is sensitive to resistivity variations in the underlying strata configuration and this is used to determine the nature of the subsea structure. In order to successfully map subterranean resistivity variations, the orientation of the current flows induced by the source electromagnetic signal must be considered [6]. The response of seawater and subterranean strata to the source electromagnetic signal is different for horizontally and vertically flowing induced current components. For horizontally flowing current components, the coupling between the layers comprising the subterranean strata is largely inductive. This means the presence of a thin resistive layers (which is indicative of a hydrocarbon layer) does not significantly affect the detector signal at the seafloor since the large scale current flow pattern is not affected by the thin resistive layer. On the other hand, for vertical current flow components, the coupling between layers is largely galvanic (i.e. due to the direct transfer of charge). In these cases even a thin resistive layer strongly affects the detector signals at the seafloor since the large scale current flow pattern is interrupted by the resistive layer.
While it is the vertical components of induced current flow which are most sensitive to the presence of a thin resistive layer, sole reliance on these components for detecting a hydrocarbon layer is not possible without ambiguity. The effects on the amplitude signals at the detectors arising from the presence a thin resistive layer can be indistinguishable from the effects which arise from other realistic large scale subterranean strata configurations. In order to resolve these ambiguities, it is necessary to determine the response of the subterranean strata to both horizontal (i.e. inductively coupled) and vertical (i.e. vertically coupled) induced current flows [6].
An electromagnetic source such as a HED antenna generates both inductive and galvanic current flow modes, with the relative strength of each mode depending on source-detector geometry. At detector locations which are broadside to the HED antenna dipole axis, the inductive mode dominates the response. At detector locations which are in-line with the HED antenna dipole axis, the galvanic mode is stronger [6, 8, 9, 10]. Accordingly, the response of the subterranean strata to vertical induced current flows along a line between a source location and a detector location is determined by arranging the HED antenna to present an end-on orientation to a detector, and the response of the subterranean strata to horizontal induced current flows along the line between the source location and the detector location is determined by arranging the HED antenna to present a broadside orientation to the detector. Data from both geometric configurations is required.
It is therefore important when designing a practical EM survey for detecting buried hydrocarbon layers using known techniques to distinguish between source and detector configurations in which the coupling between layers is largely inductive due to horizontal currents (in which case the survey has little sensitivity to the presence of a thin resistive layer) and those in which the coupling between layers is largely galvanic due to vertical currents (in which case blocking of the passage of this current flow by a reservoir leads to a survey which is strongly sensitive to the presence of a thin resistive layer).
FIG. 1 shows in plan view an example survey geometry according to the Sinha method. There are sixteen detectors 25, and these are laid out in a square grid on a section of seafloor 6 above a subterranean hydrocarbon reservoir 56. The hydrocarbon reservoir 56 has a boundary indicated by a heavy line 58. The orientation of the hydrocarbon reservoir is indicated by the cardinal compass points (marked N, E, S and W for North, East, South and West respectively) indicated in the upper right of the figure. To perform a survey, a source such as a HED antenna, starts from location ‘A’ and is towed along a path indicated by the broken line 60 through location ‘B’ until it reaches location ‘C’, which marks the end of the survey path. As is evident, the tow path first covers four parallel paths aligned with the North-South direction to drive over the four “columns” of the detectors. This portion of the survey path moves from location ‘A’ to location ‘B’. Starting from location ‘B’, the survey path then covers four paths aligned with the East-West direction which drive over the four “rows” of detectors. Each detector is thus passed over in two orthogonal directions. The survey is completed when the source reaches the location marked ‘C’.
During the towing process, each of the detectors 25 presents several different orientation geometries with respect to the source. For example, when the source is directly above the detector position D1 and on the North-South aligned section of the tow path, the detectors at positions D5, D6 and D7 are at different ranges in an end-on position, the detectors at positions D2, D3 and D4 are at different ranges in a broadside position and the detector at positions D8 and D9 are midway between. However, when the source later passes over the detector position D1 when on the East-West aligned section of the tow path, the detectors at positions D5, D6 and D7 are now in a broadside position, and the detectors at position D2, D3 and D4 are in an end-on position. Thus, in the course of a survey, and in conjunction with the positional information of the source, data from the detectors can be used to provide details of the source electromagnetic signal transmission through the subterranean strata for a range of distances and orientations between source and detector. Each orientation provides varying galvanic and inductive contributions to the signal propagation. In this way the continuous towing of the source can provide a survey which samples over the extent of the subterranean reservoir.
The Sinha method has been demonstrated to provide good results in practice. However, it has some limitations.
Firstly, since the two modes cannot be easily separated there will generally be a level of cross-talk between them at a detector and this can lead to ambiguities in the results.
Secondly, in order to obtain survey data from both in-line and broadside geometries, the HED antenna needs to present two different orientations at each source location. This requires the surface vessel to make multiple passes over broadcast locations and can lead to long and complex tow path patterns.
Thirdly, the survey can only provide the best data possible at discrete source locations. This is because of the geometric requirements of a HED antenna survey which dictate that, at any point during the survey, data can only be optimally collected from those detectors to which the HED antenna is arranged either in-line or broadside. At other orientations, separation of the inductively and galvanically coupled signals becomes more difficult, and resulting data are less reliable. For instance, referring to the figure, when the HED antenna is at a point on the tow path directly above the detector marked D1 and on the North-South aligned section of the tow path, in-line data can only be collected from the detectors marked D5, D6 and D7, whilst broadside data can only be collected from the detectors marked D2, D3 and D4. The other detectors (for example those marked D8, D9 and D10) provide only marginally useful information at this point of the survey because of the complex mixing of the galvanically and inductively coupled modes. Furthermore, if, for example, the HED antenna is at the location identified by reference numeral 57 in the figure, which is on a North-South aligned section of the tow path, in-line data can be collected from the detectors marked D3, D8, D9 and D10, but broadside data cannot be collected from any of the detectors. Since both broadside and in-line data are required for optimal analysis, the best data possible with the square detector array shown in the figure can only be collected from points along the tow path where the source is directly above one of the detector locations.
In summary, with the Sinha method, the time during which good quality data can be collected represents only a small fraction of the overall time taken to perform a survey. Furthermore, in addition to the survey being time-inefficient, it is necessary to accurately follow a complex tow path which has to complement the detector layout, and the detectors themselves must also be carefully accurately arranged. The difficulties in controlling both the position and the orientation of a towed source antenna, coupled with this need to accurately follow a particular tow path relative to the detector grid, is one of the major sources of error in surveys of these kind. The disadvantages associated with the survey constraints imposed by the Sinha method are the price to pay for resolving the ambiguities inherent in the Statoil method.